Since its discovery in 1946, Nuclear Magnetic Resonance (NMR), has become a powerful analytical tool in studies of gaseous, liquid, and solid materials.
An NMR measurement is made by determining the energy difference between nuclear spin states. In order to accomplish this, a sample of the material in question is placed in a polarizing magnetic field and excited by applying a second, oscillating magnetic field in a direction perpendicular to the first steady state field. This is accomplished by applying oscillating energy radio frequency (rf) energy across a coil surrounding the sample. The second magnetic field is created by modulating the current in this coil to produce pulses of a defined form. This second field causes transitions between nuclear spin states whose energies are determined by the first field. The energy absorbed by the nuclei during such an excitation or emitted thereby after such an excitation provides information on the differences in energy between the spin states.
After the application of the radio frequency pulse or pulses, the nuclei of interest produce a radio frequency signal known as a free induction decay (FID), which is detectable with a receiver system, where the coil surrounding the sample is the primary detection element. Traditional Fourier Transform analysis of the free induction decay (FID) generates a frequency spectrum, which contains one or several resonance frequencies or lines. The positions and widths of these resonance frequencies or lines are determined by a number of influences, such dipolar interactions, chemical shift, scalar coupling, or quadrupolar interactions on the nuclei of interest over and above that of the primary polarizing magnetic field. In certain cases the positions or resonance shifts can be partially of completely obscured by scalar and/or dipolar interactions. The accuracy of NMR measurements additionally depends upon the physical form of the sample being studied. Highly accurate chemical shift determinations and separation of NMR lines are possible for liquid samples due to the random tumbling and rapid reorienation of sample molecules in solution. This rapid reorientation effectively causes the surroundings of the resonating nuclei to appear isotropic on the time scale of the NMR experiment. If polycrystalline, powdery, glassy solids, or the like, are studied, however, the observable lines are broadened due to different orientations of particles with respect to the polarizing magnetic field.
Various methods have been employed to reduce the amount of line broadening observed in an NMR spectrum for solid samples. The line broadening can be partially overcome by using magic angle spinning (MAS). According to this technique, the sample is rotated rapidly at an angle of 54.7 degrees with respect to the polarizing magnetic field, i.e., the magic angle. This spinning removes so-called first order line broadening caused by such factors as chemical shift anisotropy, secular dipolar interactions, and first order quadrupolar interactions.
For magic angle spinning experiments spinning rate of the sample should be high in proportion with the strength of the inter-nuclear interaction or spread of chemical shift values, and is typically achieved with spinning rates of order of 1000 to 15,000 hertz (cycles per second) or 60,000 to 900,000 revolutions per minute or 1 to 15 khz. Finally, the spin rate must be relatively stable over the duration of the experiment in order not to reintroduce line broadening in the resulting spectrum.
Devices for spinning samples at very high spinning speeds were first developed in the 1920's and 1930's for various applications including the demonstration of Bernoulli's law for science students. (J. W. Beams, Rev. Sci. Instr., 1, 667 (1930); J. W. Beams, J. Appl. Phys., 8, 795 (1937); E. Henriot and E. Huguenard, Compt. Rend, 180, 1389 (1925); W. D. Garman, Rev. Sci. Instrum., 4, 450 (1933)). The use of these devices in NMR experiments was published first in the late 1950's (E. R. Andrew, A. Bradbury and R. G. Eades, Nature, Lond., 182, 1659 (1958); I. J. Lowe, Phys. Rev. Lett., 2, 285 (1959)).
Subsequentally there were a number of improvements to the basic spinner assembly to increase both sample spinning stability and/or sample spinning speed published both in the scientific literature (E. R. Andrew, L. F. Farnell, M. Firth, T. D. Gledhill and I. Roberts, J. Mag. Res., 1, 27 (1969); R. G. Pembleton, L. M. Ryan and B. C. Gerstein, Rev. Sci. Instrum., 48, 1286 (1977); S. I. Opella, M. H. Frey and J. A. DiVerdi, J. Mag. Res., 37, 165 (1980); K. W. Zilm, D. W. Alderman, and D. M. Grant, J. Mag. Res., 30, 563, (1978); B. Schneider, D. Doskocilova, J. Babka and Z. Ruzicka, J. Mag. Res., 37, 41 (1980); V. I. Bartuska and G. E. Maciel, J. Mag. Res., 42, 312 (1981); F. D. Doty and P. D. Ellis, Rev. Sci. Instrum., 52, 1868 (1981); R. Eckman, M. Alla, and A. Pines, J. Mag. Res., 41, 440 (1980), K. W. Zilm, D. W. Alderman, and D. M. Grant, J. Mag. Res., 30, 563 (1978)) and in U.S. Pat. No. 4,511,841 to Bartuska et al., U.S. Pat. No. 4,456,882 to Doty, and U.S. Pat. No. 4,739,270 to Daugaard et al.
The accurate measurement of sample spinning speed is important for a number of reasons. First, unless the sample is being spun extremely quickly at the magic angle in proportion to the nuclear interactions that are being minimized, the NMR frequencies being observed will be modulated by the sample spinning and will appear as spinning sidebands in the NMR spectrum. These sidebands must be identfied as such so that they are not interpreted as legitimate NMR resonances. Second, while spinning sidebands can be used in identifying the sample spinning speed after a spectrum has been accumulated, it is desirable to know what the spinning speed is prior to start of the experiment particularly when the sample is spinning at the high extremes of a probe's spinning speed specifications.
Non-NMR methods used for measuring for sample spinning speed, which have been described in the literature, are optical and audio methods. A portion of the sample spinning speed is in the range of audio frequencies detectable by the human ear. Audio detection of sample spinning speed in a high speed spinning device has been reported by Beams (J. W. Beams, Rev. Sci. Instr., 1, 667 (1930)) and later in conjunction of the use of these high speed spinning devices in the context of NMR experiments by Andrew et at. E. R. Andrew, L. F. Farnell, M. Firth, T. D. Gledhill and I. Roberts, J. Mag. Res., 1, 27 (1969)). The methods described by Andrew et al. consisting of listening to the charateristic fundamental audio note emitted by the rotor and beating it against a note from a loudspeaker fed from an audio signal generator. The authors found that at rates of rotation below about 500 Hz, a high frequency audio note at N times the fundamental is heard where N is the number of flutes on the rotor. At higher rates of rotation the note passes beyond the audio range of the human ear and the fundamental note equal to the rate of rotation is then heard. The beat note between this fundamental and the tone from the loudspeaker provided a sensitive indication of the constancy of rotation. Above a spinning speed of 6000 hertz this method becomes ineffective due to the limitation of the human ear.
Audio detection and monitoring of sample spinning has been abandoned in favor of detection of sample spinning via optical methods. Methods are known for detecting reflected light from a sample container to determine the sampie spinning speed. It is additionally known to use optical signals from the spinning samples with computer controlled spinning devices, which automatically regulate the sample spinning speed during an NMR experiment. (T. D. Maier and T.Huang, J. Mag. Res., 91,165 (1991); J. N. Lee, D. W. Alderman, J. Y. Jin., K. W. Zilm, C. L. Mayne, R.J. Pugmire, and D. M. Grant, Rev. Sci. Instrum., 55, 516 (1984); H. J. M. DeGroot, V. Copie, S. O. Smith, P. J. Allen, C.Winkel, J. Lugtenburg, J. Herzfeld and R. G. Griffin, J. Mag. Res., 77, 251 (1988)), Varian data sheet: Spinning Speed Control in MAS Experiments.
There are many reasons for the predominence of optical detection of sample and the abdandonment of audio detection of sample spinning speed. Foremost, the ease of implementation of optical detection techniques has caused the use of audio signals generated by the spinning sample to be ignored. The analysis of the audio frequencies of a spinning rotor have been relegated, therefore, to the analysis of the ear of the NMR spectrometer operator. Additionally, the presence of a polarizing magnetic field in which an NMR probe is subjected to radio frequency irradiation precludes the use of most conventional audio signal transducers, because they contain magnetic or electromagnetic components. Recent successful uses of piezoelectric devices for the measurement of vibrational frequencies in the range of those commonly generated by spinning samples in solid state NMR applications have been published recently. One application has been the use of piezoelectric tranducer to record the spinning speeds of two rotors in a double rotating probe (A. Samosen and A. Pines, Rev. Sci. Instrum., 60, 3239 (1989). A second application (S. I. Putterman, Scientific American, February, 1995, p.46.) demonstrated the use of a piezoelectric transducer in monitoring high frequency sounds emitted by collapsing air bubbles in sonoluminescence experiments. Piezoelectric audio transducers by their nature should be operational in a polarizing magnetic field. The piezoeletric transducer derives its action from the relations found in certain crystals or specially treated ceramic materials between a mechanical strain of the piezoelectric material and the potential differences existing on conductor plates sandwiching the material.
Even though most spectroscopy practitioners use optical detection methods for determining the spinning speed of samples spinning at the magic angle, the experienced practitioner additionally listens to the quality of sound produced by spinning sample to determine if the sample is spinning correctly.
The easiest samples to pack and spin in magic angle containers or rotors are powdered materials. When the technique of magic angle spinning of samples was being developed, powdered materials were studied exclusively because these materials were relatively easy materials to pack and spin in a rotor, particularly as the initial designs of the magic angle spinning probe were somewhat difficult to spin even sometimes even without a sample. As the magic angle probe design improved, the ease of spinning conventional powder samples led to the interest of studying samples which were of less conventional such as beads, gels and other forms. These nonconventional samples require much more care in packing so that they spin properly. A sample, which is not spinning properly can exhibit several behaviours. It can occasionally "touchdown", that is temporarily oscillate in some manner and after a period of time correct itself to a proper spinning state. Further, the sample can "crash", that is stop spinning completely or spin/gyrate/oscillate in non-recoverable fashion. Additionally, all samples including well behaved powders are suceptable to sample "touchdowns" or "crashes" if samples are spun at extremely high spinning speeds. The optical signal recorded during a "touchdown" or "crash" provides less information than the audio signal. Depending on the motion of the rotor during a "touchdown" or "crash", in some cases the numerical readout of the optical signal can be extremely oscilatory; in other cases the numerical reading of the spinning speed can appear quite stable although the numerical reading is of no relation to actual sample spinning speed. To the experienced practitioner it is quite clear from the audio signal produced by the spinning sample the the sample has had a "touchdown" or "crash". The novice user with more limited experience or a hearing impaired user may not be capable of diagnosing with only his/her ears an incorrecly spinning sample.
This invention pertains to use of audio signals in a useful and automated fashion in: (1) monitoring the spinning frequency of the sample during the NMR experiment, (2) adjusting the bearing air if a "touchdown" or "crash" of the rotor occurs, and (3) a shutdown of the sample spinning and termination of the NMR experiment if the above described actions are unsuccessful. This invention pertains to the use of audio transducers, which are compatable with their use in polarizing magnetic fields and to radio frequency irradation for sample in said magnetic fields.
This invention is a benefit to both experienced and novice users of magic angle spinning NMR probes, because it allows for the use of the spectrometer in an automated fashion.
This invention is of great importance to both the experienced user and the novice user by preventing damage to the magic angle spinning probe when the sample is both rotating and precessing due to a rotor "crash" by terminating the gas supply to the spinning sample.